The Shallow Sky

Beware the Tides of March! (Well, April)

Akkana Peck

“The RASC handbook notes, ‘Large tides’ ... I’ve been doing some tide pool exploring recently, so I had planned to end this column with a suggestion that you play hooky from work and go check out some starfish.”

As we move into April, Saturn continues to dominate the evening sky. It has moved a bit farther north, so it’s transiting higher now. You probably saw the weird and wonderful Cassini photo last month, where the spacecraft moved into a polar orbit and took a shot looking down on the planet, showing the shadow of the planet streaming out across the rings. You won’t see anything quite like that with your telescope — but take a look anyway. There’s plenty of detail to be seen!

If you get tired of the rings and want more of a challenge, try going after Saturn’s weird and distant moon Iapetus. It’s weird because one side is much darker than the other. When the bright side is facing us, Iapetus is an easy magnitude 10.1 object; but the dark side is only magnitude 11.9. April 14 is a good time to see the bright side. Grab a finder chart from Jane Houston Jones’ page at http://www.otastro.org/iapetus/ and compare its brightness to Saturn’s other moons and some nearby stars. Then check back on May 24th, when Iapetus’ dark side will be pointed our way.

Meanwhile, Venus continues its dazzling early evening run. On April 18 it skims past the Hyades cluster, overwhelming those relatively faint stars.

The rest of the planets are clustered in the morning sky. Mars and Mercury hover low in morning twilight — wait a few more months if you want to see much of them. Jupiter is better: it rises late in the evening and is visible until dawn. Pluto rises at about the same time as Jupiter, which means it will get high enough for a dedicated morning Plutocrat to find it — but most of us will do better waiting a few months. Uranus and Neptune hide down in the twilight with Mars.

Occultation watchers who have been frustrated at all the recent occultations elsewhere in the world finally get a break: a lunar occultation of Regulus on April 26. The bad news? It’s in the wee hours of the morning, just short of 3 am. On the 16th, a slim crescent moon passes close to the Pleiades, continuing an ongoing series of pretty moon/Pleiades encounters.

Then the next day, April 17th, the moon crosses perigee — the point of its orbit where it’s closest to Earth — just a few hours before new moon. The RASC handbook notes, “Large tides”. That note caught my eye because I’ve been doing some tide pool exploring recently, so I had planned to end this column with a suggestion that you play hooky from work and go check out some starfish.

Then I checked a tide table. First problem: that low tide is at 5 am, and most of us aren’t quite up for tide pooling at that hour. But the second problem was the kicker: the low tide isn’t all that low on the 17th. But it gets lower the following day, and even lower two days after perigee. No April fooling!

How can that be? Well, tides are a lot more complicated than the simple equatorial bulge you learned about in Astronomy 101.

I’m sure you know the basics. The moon’s gravity pulls hard on the near end of the Earth, and less hard on the far end. This causes a bulge pointing toward the moon, and another bulge pointing away. (That last part sounds counter intuitive at first, but think of the moon pulling harder on the center of the Earth than it does on the far end, so the whole thing gets stretched like a rubber band. It’s not that the moon is pushing the water on the opposite side of the Earth, but that the moon is pulling the Earth more than it’s pulling that far–away water).

Then add in the sun, which causes a similar effect, though only about half as strong as the moon’s, because it’s so much farther away. When the sun and moon are stretching the Earth in the same direction, like at new or full moons, you get a “spring” tide — as in “spring forth”, not the season after winter. When they’re pulling at right angles to each other, you get small tides, called “neap tides”.

All very well, but why are the highest high tides and the lowest low tides several days after new moon? Even weirder, if you check tide tables for places around the world, you’ll find a few places, like the Coral Sea, where the extreme tides actually happen *before* new moon! What’s up?

To understand that, first imagine the Earth as if it were completely covered with water, no continents at all. As the planet rotates beneath the moon during the course of a day, whatever’s under the moon is always being pulled upward to a high tide. But that point changes as the Earth rotates. The result, in our ocean–only Earth, is a huge traveling wave of water — a tidal wave (and no, for once that doesn’t mean tsunami) — that follows the moon around the earth at just under a thousand miles an hour.

But in practice, that can’t happen in most of the real world’s oceans: the wave smashes into a continent before it can get very far. In addition, the oceans aren’t deep enough to allow for a wave that big and that fast. So in practice, you get waves reflecting off continents, and interference with features on the ocean floor. Throw in a handful of other variables like the Coriolis effect and you get a complicated mess.

Long ago, oceanography books used to teach that the tides all originated in a band around 50 degrees south in the Southern Ocean, the only place where a tidal wave can roll around the earth unimpeded with no continents to get in the way. The delay between new moon and the highest high tide — called “the age of the tide” — was said to be due to the time it took for the tidal wave to propagate from the Southern Ocean.

You’ll still find that theory in a few places — like wikipedia. But most newer sources seem to discredit the theory. And indeed, it’s hard to imagine that the tides we see here aren’t at least partly due to the influence of the moon and sun on the Pacific Ocean. It also doesn’t explain the Coral Sea.

So what’s the modern theory explaining the age of the tide? As far as I’ve been able to tell, there isn’t one.

You can measure the age of the tide for specific locations, and you can even try to calculate the contributions of the various components: the diurnal variation caused by the declinations of the moon and sun, the contributions from reflections off the coast, the local ocean floor profile, and a host of other factors. But as far as a general theory for the age of the tide ... most sources just avoid the question.

So when you see “large tides” mentioned in the RASC handbook, don’t take it too seriously! And if you want to go tide–pooling, use a tide table ... not a moon chart.